Gun is a weapon that fires a bullet, a shell, or some other projectile. Most guns fire by the force of a gas created by the rapid burning of gunpowder.
Guns are classified according to size. Hand-carried guns, called small arms, include pistols, rifles, and shotguns. Portable, automatic weapons that can fire from 400 to 1,600 rounds of ammunition per minute are called machine guns. Large guns are called cannon or artillery. They may be stationary, as on ships, or mounted on wheels or on self-propelled carriers.
Gun size is expressed in terms of caliber, the inside diameter of the gun’s barrel, measured in inches, hundredths of an inch, or in millimeters or centimeters. A .45-caliber revolver has a barrel with an inside diameter of 45/100 of an inch (11 millimeters). A 75-millimeter cannon has a barrel with an inside diameter of 75 millimeters. Most guns have spiral grooves called rifling on the inside of the barrel. The rifling gives a spiraling motion to a bullet or shell. This motion helps prevent the bullet from wobbling, and helps improve accuracy and increase range.
No one knows who invented the gun. Evidence in China shows that handheld cannonlike weapons may have been used there in the A.D. 1100’s. The Arabs of North Africa used similar weapons in the 1300’s. These early guns consisted of a brass or iron barrel with a small hole at the closed end for igniting the gunpowder.
